1. Field of the Invention
This invention relates to an image processing apparatus that processes images using digital image signals, and more particularly to an image processing apparatus capable of preventing the moire phenomenon that occurs when the number of gradations of the image data in dot areas is reduced.
2. Description of the Related Art
In a conventional image processing apparatus, the attribute of the image is first identified (i.e., character area or photograph area) to enable processing appropriate to such attribute to be carried out. Specifically, processing focusing on resolution is carried out regarding a character area, while processing focusing on gradation is performed regarding a photograph area. In such attribute determination, a process that uses the density difference in M-level image data is widely used. This technology uses the characteristic fact that a character area includes substantial edge components (i.e., that the density difference is large; see FIG. 12), and a photograph area has a flat density distribution (i.e., that the density difference is small; see FIG. 13).
However in a dot area, moire may occur due to interference between the resolution for image reading and the dot frequency of the original document. When moire occurs, the image quality deteriorates. Even if this moire is not conspicuous in an M-level image, when the image is converted into an N-level image (M>N), i.e., when the number of gradations of the image is reduced, it becomes markedly visible. Therefore, an N-level conversion process that prevents the occurrence of moire has been desired.
In the conventional image processing apparatus described above, however, the problem exists that moire becomes conspicuous in dot areas after N-level conversion (i.e., when the number of gradations is reduced), because, while it is desirable to preserve linear gradation in dot areas, such preservation is not possible when processing that uses the density difference is used. In other words, as shown in FIG. 14, a dot area is judged to be more character-like than a character area as the read resolution increases, and processing focusing on resolution is carried out on the dot area. In addition, because dot areas indicate various density patterns depending on such conditions as the screen angle, density and read resolution, it is difficult to accurately identify a dot area via processing using density difference. That is one of the reasons that appropriate processing cannot be performed on dot areas. Because N-level conversion that preserves linear gradation cannot be carried out regarding dot areas, as described above, moire visually stands out in such areas.